Configuration of the conventional type of fish finder is, for instance, as shown in FIG. 15. In this figure, the conventional type of fish finder comprises a transducer 101, a transducer driving circuit 102, a transducer receiving circuit 103, a comparator 1504, a microcomputer 1505, an attenuator 1506, and a display unit 108.
According to a control signal from the microcomputer 1505, a trigger pulse is transmitted from the transducer driving circuit 102, and a supersonic wave is transmitted, for instance, into sea water from the transducer 101 in response to said trigger pulse.
When a supersonic wave echo from an obstacle under water or the sea bottom is received by the transducer 101, a receiving signal having a voltage level corresponding to the amplitude of the supersonic wave echo is outputted from the transducer 101, this receiving signal is detected and amplified in the transducer receiving circuit 103, and then supplied to the comparator 1504.
In the computer 1504, signals each having a voltage less than a specified value are removed, and the remaining signals are supplied as digital signals each consisting of "1 and 0" to the microcomputer 1505. The microcomputer 1505 provides displays of a dot pattern or an icon on the display unit 108 according to the received digital signals.
Generally, underwater scan by a fish finder is executed in the underwater measurement mode for detecting obstacles or the like under water and in the sea depth measurement mode for detecting the sea bottom, the two modes being switched repeatedly.
Basically, it is known that a supersonic wave echo from the sea bottom is far larger than that from an obstacle or the like under water, and determination in the sea depth measurement mode as to whether a supersonic wave is from the sea bottom or from an obstacle or the like is executed by making use of reflection characteristics. Namely, an attenuation rate in the attenuator 1506 is changed during a certain scanning period under control by the microcomputer 1505, an amplification gain in the transducer receiving circuit 103 is gradually raised from "0", and a supersonic wave echo first received by the microcomputer 1505 (when a prespecified level is surpassed in the comparator 1504) is determined as that from the sea bottom.
In the underwater measurement mode, measurement is executed with an amplification gain in the transducer receiving circuit 103 fixed at a constant value. However, the amplitude of a supersonic wave echo changes according to type of transducer or underwater conditions to be detected, so that attenuation rate of the attenuator 1506 can manually be adjusted by a user. For this reason, to carry out correct measurement, it is necessary for a user to adjust attenuation rate of the attenuator 1506 empirically or by trial and error.
As described above, in the conventional type of fish detector, it is necessary to repeat scanning to obtain an appropriate attenuation rate, and also it is necessary to make the attenuation circuit larger and increase scanning times for obtaining a higher resolution of adjustment level. This impedes device minimization or realization of higher operational speed.
Also in the conventional type of fish finder, there are problems that a display showing a result of detection can not be obtained as a clear image because of influence by attenuation of supersonic waves during propagation, foreign materials in or contamination of sea water, or air bubbles or the like, and that the condition of the sea bottom can not be detected correctly.